1. Technical Field
This invention involves a system and a method for using ultrasound to sense the speed and direction of flow of a fluid, such as blood in a coronary artery.
2. Description of the Related Art
The measurement of blood flow in the coronary arteries is a well-known technique for diagnosing coronary artery diseases. There are, consequently, many different devices and methods for determining this blood flow.
One common sensing technique involves the use of ultrasound. Using this technique, ultrasound is directed into the body of the patient and tiny particles such as red blood cells, which are suspended in the blood, scatter the ultrasonic energy back towards the transducer. The transducer then converts the back-scattered ultrasonic energy into an electrical signal that is processed in some known manner to determine an estimate of the flow.
One great advantage of ultrasonic sensing is that it is non-invasive, meaning that it can be carried out without having to cut or insert anything into the patient's body. A problem one faces when using existing ultrasonic flow measurement techniques, however, is that measurements are often made through the "keyhole" between the ribs in a transthoracic scan, where the coronary arteries typically twist over the curved surface of the moving heart wall. The direction of the blood in the arteries or the motion of the heart wall with respect to the line-of-sight of the ultrasonic beam is therefore usually not known. This is a serious problem for the many common techniques that use the principle of Doppler shift.
The Doppler principle used in existing techniques for calculating flow velocity v based on the frequency shift of ultrasonic waves scattered by moving red cells can be expressed as follows: ##EQU1## in which f.sub.0 is the frequency of the ultrasonic wave sent into the body, v is the flow velocity, c is the speed of sound, .theta. is the angle between the line-of-sight direction of the beam and the flow, and f.sub.d is the detected frequency shift of the signal that returns to the transducer. As long as cos .theta. is not equal to zero, the frequency shift will increase with increasing flow velocity.
As the equation shows, it is not possible using conventional Doppler techniques to detect any frequency shift if .theta. equals 90.degree., that is, if the flow is perpendicular to the line-of-sight of the ultrasonic transducer, regardless of how fast the blood is flowing. Police officers who use radar guns to check for speeders are a more common example of this problem: the officers cannot position themselves at right angles to the cars being checked because the typical radar gun uses the same Doppler principle and would tell the disbelieving police officers that the cars were not moving at all.
If the direction of flow is at an angle of 60.degree. from the line-of-sight of the ultrasonic transducer, the indicated frequency shift will be only half what it would be if the flow and the line-of-sight were parallel. In general, the angle .theta. is not known beforehand. In the context of blood-flow measurements, what is needed is therefore a technique that is substantially isotropic, that is, direction-independent. Alternatively, a technique is needed that determines the direction of flow so that Doppler measurements can be adjusted accordingly. Furthermore, a system is needed that can implement these techniques. This invention provides such techniques and such a system.